Use the artboard. In the Export dialog check the box at the lower left. You can also select among multiple artboards if you have them. To export jus a portion of the artwork, draw a box and go to Object>Artboards>Convert to Artboard and use the number of the new artboard for the export.

Did you try what I told you? This is the replacement for the Crop Area from CS3. Are you working on an artboard which is the exact size you need for saving or do you need to crop the artboard area tighter? What format are you saving to?